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...and in another [Clip #10] the gun shot sound with a separate instrument playing "twang". But sorry! I did not hear them.ST

I don't want to move forward unless I can demonstrate the 'twang' to you, that is, the tonality in the reverberation which is definitely not present in the dry gunshot clip.

I recommend headphones to listen to any of these clips. Let me increase the loudness. BEWARE! These sounds could damage your speakers, headphones or hearing. The gunshots were previously at about 80% of full scale: now to 100%, maximum loudness.

(Clip #9A) Dry gunshot, gain increased to 100%

Clip #9A above is completely dry. No hint of any tonal character in the impulse. Now if we take the starting pistol inside and fire it four times again in an untreated room we will hear something like this (source file is Clip #9):

(Clip #10A) Gunshot in large untreated hall, gain of gunshot increased to 100%

Now if we remove the direct gunshot from the in-room recording we are left with only the reverberation. And if we turn-up the loudness of the reverberation by about 40dB (100x) the reverberation clearly has a tonality. The reverberation could be 'played' as musical instrument here:

(Clip #11) Gunshot in large untreated hall, gain of gunshot increased to 100%

The essential point is that the dry gunshot had no tonailty, but fire it in the room and now we have a clear pitch. Interesting?

Alan A. ShawDesigner, ownerHarbeth Audio UK

Comment

The starting pistol when fired in the room has (I hope you can hear now) now taken on a sonic signature of the room, a definite pitch or tonality. In fact, there are multiple frequency components in the reverberation. We can isolate them with very sharp narrow-band filters.

Again to remind us:

(Clip #11) Gunshot in large untreated hall, gain of gunshot increased to 100%

Here is one strong frequency component isolated from within the reverberation of Clip 11, via a filter between 500-700Hz, all other frequencies are rejected:

There are multiple frequences in the reverberation, but that band is particularly strong. Remember: these energetic tones were not at all audible in the dry gunshot. They have been 'generated' by the room itself.

Next step: consider this. We've shown that if we take a gun with a clean, characterless click or crack sound when used outside and we take it inside and fire it, now we hear tones in the gunshot's reverberation tail due to something strange about the acoustics of the room and nothing whatever to do with the gun. The gun is a sonic generator. So is a loudspeaker. So if we take a loudspeaker with a pure, clean characterless sound and place it in the ordinary listening room and start it playing, what we hear will also be polluted by the room. The direct sound from the speaker to our ear sitting in the sweet spot will be the same clean sound. But immediately following that will be the reverberation of the room. What we hear at the sweet spot is therefore a combination of the speaker's clean direct (anechoic) sound + the twang from the room. Regardless of where we fire the gun in the room and regardless of where we place the speakers(s) in the room, the room is going to add its twangy contribution which will spoil out listening pleasure.

The proof of the above would be that if we reproduced the dry gunshot over the speaker(s) in the room, assuming that the speakers were basically of good quality, the speaker-gunshot's reverberation would exhibit exactly the same twangy character as the real gunshot.

Are we ready to move forward to the next step now? Tell me if not please.

Comment

Firstly, this is a fascinating thread which I am just catching up with and which has already started to explain something that has puzzled me for years. Thank you.

Secondly - this may well be a pedantic point but can I ask anyway? The gunshot is described as a "clean, characterless click or crack" but that does not mean it is without pitch does it? I realise that 'pitch' is a function of human perception but the sound will nonetheless have a frequency won't it?

I know that this is not the point that was being made but the reason for asking is that I actually found clip 12 rather startling in the sense of being surprised that that component was present in the total echo sound, ie had it not been isolated I would not have 'heard' it. I do take the point that it was not present in the original gunshot.

My current way of thinking about this is that, visually, the characterless sound is grey - an equal balance of colours. The 'tonal character' comes about when we move away from grey.

Comment

...The gunshot is described as a "clean, characterless click or crack" but that does not mean it is without pitch does it?

There is no character at all in the click, as generated. The click is an impulse, and certainly that impulse contains all frequencies in equal proportion right across the audio band. It is as pure and characterless as truly random white noise, the type of static noise you hear between stations on an FM radio.

Therefore as generated*, the impulse I've used has definitely no pitch, because it has no resonance generators and to sustain a pitch (a note). You have to have some sort of resonant system generating and sustaining the pitch. (A musical instrument is nothing more or less than a resonator in the same way that an empty wine bottle is a resonator and produces a pitch when blown across).

It is of course a trivial matter to put the click through a spectrum analyser and have a look at its spectral make-up. However - and this is important to bear in mind - you are hearing the click through your sound card and speakers/headphones as you cannot sense the click directly in your brain (as yet). So your perception of the click is the digitally generated clip mixed with the resonance characteristics of your speakers. Remember the box in the box in the box idea? The clean, dry click takes on the character of the speaker box (and drive units/crossover) and is being reproduced in your listening room - another box. So it is entirely possible that to your ears, with your speakers in your room that the click has some sonic character, but this is not inherent in the click itself for sure. So the very evident twang in the reverberation tail in Clips 11, 11A and 12 is solely that of the room modifying the characterless click.

* Note: when the repetition rate of the click starts to become very frequent, that is, very many times per second, it may well take on a pitch, but this is not relevant at this point. I've shown that even when there is a gap of a second or two between the clicks, there is still a definite audible character in the reverberation.

Alan A. ShawDesigner, ownerHarbeth Audio UK

Comment

... It [the click] is as pure and characterless as truly random white noise, the type of static noise you hear between stations on an FM radio. Therefore as generated*, the impulse I've used has definitely no pitch, because it has no resonance generators and to sustain a pitch (a note). ...

The advantage of the gunshot click is that is discrete. That means that it is not continuous (like a machine gun) but occurs just every second or two, as we generate it. But whilst the energy in the click is all brought together into one instant, we can generate the same audio spectrum from white noise.

To remind ourselves here is the click again:

(Clip #9A) Dry gunshot, gain increased to 100%

And here is exactly the same frequency spectra but spread out in time:

Incidentally, we can use the pulse or the random noise interchangeably for making acoustic measurements on speakers, microphones, rooms, amplifiers CD player or whatever. Under controlled conditions, the results would be identical. But, as already noted, the click have very little acoustic energy in it so if we were making acoustic measurements with a microphone we would have to ensure that the environment was extremely quiet or even the slightest ambient noise would be picked up by the microphone and would garble the measurement. So for practical acoustic (environmental) measurements, the pulse is not really much use. White noise would be a better bet because we can push so much more power into the test, so that we can swamp ambient noise like doors shutting or cars and planes passing by.

I've taken the white noise Clip 13 and added the hall reverberation that gave us Clip 10A, 11 & 12. Now we can't hear the twang but it is definitely there buried under the random white hiss. It sounds like this now:

(Clip #14) White noise with hall reverberation added

Although we can't immediately identify it, the twang in Clip 14 is the same as in Clip 12 here:

The point to make is that the listening room may well have a serious sonic character. That may be masked by the type of music being played or it may be exposed by the music. It may irritate you immediately or it may take hours, days or weeks until your subconscious alerts your conscious to the presence of a drone from the room. But once you are sensitised to the twang, you just can't get it out of your mind and your enjoyment of music is seriously impaired.

Comment

Just catching up with this before bedtime - have been rediscovering old favourites via Spotify so computer speakers are at music-listening level (and those white noise clips are pretty loud). So when I hastily stopped the replay of the basic white noise clip, it was suprising (but not too much) to hear a "raaa" echo once it had stopped. BTW the gunshot doesn't excite the room in the same way.

{Moderators comment: the white noise is actually at a much lower peak level than the click. But because the white noise is continuous it seems louder.}

but if we abruptly truncate (sharply switch off) the white noise generator, we can expose the hidden reverberation here:

(Clip #15) Truncated white noise with hall reverberation exposed

Now if we pass that reverberation tail through our 500-700Hz filter as we did with Clip 11 and apply some boost so we can clearly hear the reverberation we have clearly exposed the room twang from under the random noise:

Regardless of whether we use a single-pulse click or gun shot or continuous noise (or music) we can expose the characteristic sonic signature of the room by analysing what the microphone in the room hears

Regardless of the stimulus, the signature of the room has exactly the same pitch and importantly ...

Onlyone single clickis required to energise the room, to set it reverberating and expose its characteristic twang

The implication of 3. is that from the very first note of the very first bar of the music leaving the speakers and radiating into the room, the room is given enough energy to set it into resonance, and once the characteristic twang is established in that first 0.1 second, every successive note will re-energise and re-stimulate the room's character until some time after the speakers have fallen silent after the final musical note. Therefore, an untreated, undamped room will add its sonic signature as a (continuous) drone over every note and every bar of the music. That is anathema to high fidelity sound reproduction. We cannot expect high fidelity sound reproduction is untreated, undamped rooms. And furthermore, it must be extremely obvious that gadgets and gizmos of small surface area proportionate to the room's surface area cannot and will not make any improvement whatsoever to the room's acoustics, and if they are bell-like in shape, material and construction may well degrade the acoustics even more at their resonant frequency.

So where do we start analysing the room and making a treatment plan to ameliorate uneven reverberation - that is, reverberation that has a strong sonic signature. And more pertinently, how can we reliably 'test' the room without any test equipment?

Alan A. ShawDesigner, ownerHarbeth Audio UK

Comment

.....And furthermore, it must be extremely obvious that gadgets and gizmos of small surface area proportionate to the room's surface area cannot and will not make any improvement whatsoever to the room's acoustics, and if they are bell-like in shape, material and construction may well degrade the acoustics even more at their resonant frequency............So where do we start analysing the room and making a treatment plan to ameliorate uneven reverberation - that is, reverberation that has a strong sonic signature. And more pertinently, how can we reliably 'test' the room without any test equipment?

What should we use? Clapping hands, knocking a glass with a spoon, two soup spoons by knocking their bottom making a nice "tuk" tuk" sound. I used all these and including test tone CD with a single speaker.

ST

Comment

So where do we start analysing the room and making a treatment plan to ameliorate uneven reverberation - that is, reverberation that has a strong sonic signature. And more pertinently, how can we reliably 'test' the room without any test equipment?

Since the reverberation is a physical effect resulting from physical properties of the room, is there some way of getting from those properties to the "problem" frequency band? (this is discounting reverberations in the structure of the room).

It may be a bit early to ask the question, but it's intriguing to think about what physical process is at play here. A first guess would be interference between room modes... but that's probably barking up the wrong tree...

Comment

... Clapping hands, knocking a glass with a spoon, two soup spoons by knocking their bottom making a nice "tuk" tuk" sound....

As we discovered when listening to the impulse (the click) there really isn't much energy in it. So I'd forget about the two spoons and the wine glass. We really need to put more power into our test impulse to be able to hear anything significant in the echo. So as you suggest, clapped hands are a better option.

But exactly how to clap the hands? We want to generate as clean, sharp characterless crack as we can. We can try two approaches: Generating a clap as we would when applauding, whereby we bring together the cavities of our hands into a clasp and that produces a lot of low-frequency energy as the air is squeezed out from the void formed by the palms. Alternatively, we can keep the two palms flat and parallel and bring them smartly together. The sound will be different so the acoustic spectra will be different. But for sure, it's far less painful to bring the cupped palms together than suffer the sting from the underside of the fingers brought together! Either way, we need to repeat the clap as loud as we can about once every two or three seconds, and we need to walk around the room carefully listening to the echo and defocusing on anything in the room. Our total attention should be to the character and direction of the reverberation.

So the next step is to actually do some walk-clap-listening (WCL) in your listening room (or in any building) and to build-up some expertise for yourselves as to how different rooms respond to the WCL test. At first, don't expect to be able to hear much in character in the echo - but with a little more experience and comparing the sound in different parts of the room, you'll really see begin to appreciate how very influential the room acoustics are to the reproduction quality of high fidelity sound at home. If you want a quick comparison of extremes, try the WCL test in your bedroom and then your bathroom. And just what factors make these to extremes behave as they do acoustically?

The listening room's acoustics is the hidden devil in the hi-fi reproduction chain. Unless this devil is tamed, the listener is just wasting his money on ever more sophisticated equipment.

Comment

I'm going to have to assume that you're with me on this and move on. I think the audio examples are clear enough.

We've looked at the general reverberation in the concert hall and/or listening room. We've seen that if the general acoustic takes on a sonic character, that that character overlays reproduced sound in the room. We've seen from the white noise (random noise) clips that even when the room drone is quite marked, we may not be able to hear it consciously. We may only latch onto that acoustic issue when the music abruptly ceases or between the notes under ideal conditions. We can be absolutely certain that the room drone is not part of the composer's score, nor is it a characteristic of the speaker, and to spend serious money on hi-fi equipment just to to dump it into an untreated listening room and expect great sound is just daft. Great painters need natural, untainted daylight to reproduce reality: great audio needs natural, clean acoustics in which to perform.

Aside from the drone or honk or twang we've heard in the room's reverberation there are other listening room (or studio) acoustic issues which are at least as serious, and may be even more irritating. One of these is sonic bounce of flutter. Sound waves behave just like a tennis ball, and given just the right circumstances they will efficiently bounce backwards and forwards between (generally opposing) walls. With each bounce they'll lose a little energy (just as the ball would) until finally they are too weak to make the final journey and the echo ceases. But the sonic character of the bounce is really irritating because it takes on a hardness of tone which may be unrelated to the music. So it is more audible.

Here is the gun shot outside again:

[Clip #9A again] Dry gun shot

And next is the gun shot (click) inside and where it excites a bounce echo, probably between two opposite walls. Note, I have not increased the echoes level relative to the click and you can hear that the echo is really prominent, and has the same sort of sharp wide-band tonality as the gun itself. Fatiguing to listen to.

[Clip #17] Gun shot inside, bounce echo

First the music as before, recorded dry, in the anechoic chamber:

[Clip #4 again] (Dry music, recorded in dead room)

Now we add the same bounce echo used to make Clip 17 to the music and we have this:

[Clip #18] (Bounce echo added to dry music)

There are three things to listen for in Clip #18.

The bounce echo make the string tone brighter and more toppy through

In the middle of the clip, between the beats, you can hear a distinct echo as if there are a greater number of string players, but slightly out of step

The final seconds have a definite wah-wah modulation effect and a 'dwaaannnggg' due to an unwelcome coincidence between the tempo and the timing of the echo and its character.

All in all, a horrible, fatiguing sound fairly typical (although this is extreme) of home listening rooms.

Now we can compare Clip 18 above with its bounce echo with Clip 19, where I've only added the previous hall echo what gave the coloration from clip 10 onwards.

[Clip #19] (Hall echo added to dry music)

What sounds worst? Clip 18 or Clip 19? Clip 19 is of the music played in the great hall or Clip 18's bounce echo of the type that we'd find in a typical domestic room? I'd say Clip 18 was far more fatiguing to listen to. At least we are generally familiar with and untroubled by the (relatively) long reverberation of large halls. But strong, intense echoes rapidly following the notes are disturbing.